1. Field of the Invention
This invention relates to the exploitation of modified pigments for laser-induced thermal transfer.
2. Description of the Related Art
Thermal transfer processes such as, for example, dye sublimation, dye transfer, melt transfer, and ablative material transfer, are well known in applications such as color proofing and lithography. These processes often employ a donor element that includes a layer of material to be transferred (“transfer layer”), and a receiving element that includes a substrate for receiving the transferred material (a “receiving substrate”). The donor element and the receiving substrate are brought into close proximity or direct contact with each other, and portions of the donor element are heated to transfer the corresponding portions of the transfer layer to the receiving substrate. Heat may be generated using a heating element (e.g., a resistive heating element), converting radiation (e.g., light) to heat, and/or applying an electrical current to a layer of the thermal transfer element.
In a digital transfer process, the exposure to radiation takes place only in a small, selected region of the assembly of the donor element and receiving substrate at one time, so that transfer of material from the donor element to the receiving substrate occurs in a patterned manner. Thus, a non-patterned donor is in this manner selectively transferred to a receiving substrate in a patterned manner. Computer control facilitates high resolution and high speed transfer. Alternatively, in an analog process, the entire assembly may be irradiated and a mask may be used to selectively expose desired portions of the thermally imageable layer. See, for example, U.S. Pat. Nos. 5,857,709 and 5,937,272.
Patterning materials using thermal transfer processes is generally faster and less expensive and can provide greater resolution than patterning by using photolithographic processes. Thermal transfer using light can also provide better accuracy and quality control for very small devices, such as small optical and electronic devices, including, for example, transistors and other components of integrated circuits, as well as components for use in a display, such as electroluminescent lamps and control circuitry. The size and shape of the transferred pattern (e.g., a line, circle, square, or other shape) can be controlled by, for example, selecting the size of the light beam, the exposure pattern of the light beam, the duration of directed beam contact with the thermal transfer element, and the materials of the thermal transfer element. Moreover, thermal transfer using light may, at least in some instances, provide for better registration when forming multiple devices over an area that is large compared to the device size. Methods and devices for performing light-induced thermal transfer are known to those of skill in the art and are described in U.S. Pat. Nos. 6,194,119; 7,108,949; 6,921,614; 5,523,019; and 6,855,384.
Thermal transfer to pattern layers from donor elements can also be useful to de-couple layer coating steps from patterning steps, for example where such coupling can limit the types of layered structures, or the types of adjacent structures, that can be patterned. Because no solvent is required for thermal transfer, materials can be patterned that may be sensitive to the various solvents are employed in prior art lithographic methods. Conversely, materials may also be patterned without concern that solvents may adversely affect previously deposited materials. Biological materials especially may be patterned without risking denaturation of proteins or the interruption of hydrogen bonds between or within nucleic acid molecules.
In some donor elements, a separate heat generating layer is employed. The heat generating layer may be a light to heat conversion (LTHC) layer incorporating a material that absorbs a desired wavelength of radiation and converts at least a portion of the incident radiation to heat. The heat from the LTHC layer heats the transfer layer, causing the material to be transferred to the receiving substrate.
LTHC layers have employed pigments such as carbon black in polymer compositions (see, for example, U.S. Pat. Nos. 5,695,907, 5,863,860, 6,190,826, and 6,194,119). Such pigments are finely divided, insoluble, solid particles which are, in general, not readily dispersible in liquid vehicles.
Difficulties with dispersion of the pigment generates a number of disadvantages for the production of thermal transfer devices. Where a separate LTHC layer is employed, the inability to incorporate sufficient amounts of pigment into the LTHC layer can reduce the optical density of the LTHC layer, increasing the amount of light required to accomplish thermal transfer. While dispersants may be used to facilitate dispersion of the pigment, these can increase the viscosity of the dispersion. High viscosity dispersions increase the difficulty of manufacturing layers incorporating these dispersions. Known techniques for producing thin layers, e.g., microgravure printing, may not be suitable for use with high viscosity media, and thicker layers may be required to avoid pinholes and other defects. However, thicker layers result in increased materials expenses during manufacturing and also increase the amount of light required to accomplish thermal transfer, as it will take longer to heat the thicker layer. Furthermore, because it takes longer to conduct heat across the thickness of a thicker LTHC layer, heat will convect laterally along the LTHC layer, further decreasing the resolution of the thermal transfer device.
Thus, it is desirable to have thinner, smoother LTHC layers without sacrificing optical density for use in light induced thermal transfer since they can be used to deliver images with higher resolution and lower line edge roughness at lower materials cost.